When making prints from photographic negatives, it is desirable to use photographic paper which has a contrast selected to achieve satisfactory tone-reproduction of the original image based upon the contrast of the negative. Papers having higher contrasts, for example, are useful in printing negatives that themselves exhibit low contrasts, so that a satisfactory final print can be achieved. As such, photographic manufacturers offer several grades of photographic paper. In order to avoid the need for separate papers of different grades, papers possessing the ability to achieve different, selected, contrasts depending upon the wavelength of exposing light have also been employed.
Various methods for obtaining variable, or selective, contrast properties in single photographic elements have been proposed. Early proposals included photographic elements containing two distinct emulsions, such as a high contrast silver chloride emulsion which has been dyed so as to render it sensitive to green light and a low contrast silver bromide or silver chlorobromide emulsion with inherent sensitivity to blue light as disclosed in U.S. Pat. No. 2,202,026. With such a combination of emulsions, a high contrast negative could be exposed through a filter passing only blue light, to which the low contrast emulsion is sensitive, thus producing a print of normal contrast. Alternatively, a low contrast negative could be exposed through a filter passing only green light to which only the high contrast emulsion is sensitive, thus again producing a print of normal contrast.
Use of emulsions of different halide types in order to achieve differences in contrast, however, resulted in complications due to different rates of development of the two emulsions. For negatives which do not fall at either extreme of contrast, and which thus require both emulsions to contribute to the final image in varying amount, the different rates of development made it difficult to judge accurately the adjustment of development time and exposure to optimize the final appearance of a print. Image tone was also found to vary depending upon the relative contribution of each emulsion to the final print.
A second approach to providing a variable contrast system involved the use of a single type of emulsion, but the whole emulsion was not completely dyed. This approach requires the use of a sensitizing dye which itself alters the contrast of the emulsion it is used with. A portion (e.g. less than half) of the emulsion may be dyed with sufficient dye to give the silver halide grains of that portion high sensitivity to a wavelength outside the inherent sensitivity and a different contrast without exceeding the adsorptive capacity of the grains, and that portion then mixed with the undyed portion before coating as described in U.S. Pat. No. 2,384,598. Alternatively, the entire single emulsion may be dyed with a restricted amount of a sensitizing dye which controls the contrast of the emulsion depending upon the amount of the dye used as described in U.S. Pat. No. 2,280,300. A variable contrast effect is obtained as a result of an uneven grain-to-grain distribution of the restricted amount of dye which apparently results from preferential adsorption of the dye on some of the silver halide grains.
In many conventional single emulsion type variable contrast systems, an inherently blue sensitive, relatively high contrast silver halide emulsion is partially sensitized with a green sensitizing dye which also reduces the contrast of the emulsion for exposures to green light. The emulsion may also be sensitized with a blue sensitizing dye to enhance the inherent sensitivity in the blue region. Benzimidazolooxacarbocyanine sensitizing dyes have been found to be effective green sensitizing dyes for single emulsion type variable contrast photographic elements. One particular benzimidazolooxacarbocyanine sensitizing dye which has found use in prior art variable contrast photographic elements is the following dye (A): ##STR2##
Other benzimidazolooxacarbocyanine dyes useful in variable contrast elements are disclosed in U.S. Pat. No. 4,987,063 and British Patent Specification 1,390,247, the disclosures of which are incorporated by reference.
Care must be taken in the selection of spectral sensitizing dyes in order to provide the desired green sensitization while avoiding sensitization in undesired regions. For example, in variable contrast photographic elements, it is often desirable to maintain insensitivity at "safelight" wavelengths, such as longer than 570 nm. This provides a photographic element, which, while being spectrally sensitized to blue and green wavelengths, may also be handled under light within the safelight region without generating any latent image in the silver halide.
While many benzimidazolooxacarbocyanine dyes have been found to be effective variable contrast green sensitizers, many of them also have been found to be not as tolerant to safelight exposures at wavelengths longer than about 570 nm as would be desirable. It is therefore an object of the invention to provide variable contrast green spectral sensitizing dyes which produce a photographic element with better tolerance to safelight exposures at wavelengths longer than about 570 nm.